Development of conjugate vaccines to protect pigs against swine pleuropneumonia induced by Actinobacillus pleuropneumoniae

The three major virulence determinants of Actinobacillus pleuropneumoniae, the causative agent of swine pleuropneumonia, were utilized in the preparation of two conjugate vaccines that could reduce or eliminate the effects of these factors and thus protect pigs against this disease.;The two subunit conjugate vaccines were prepared by covalently coupling (a) partially depolymerized capsular polysaccharide to hemolysin protein and (b) detoxified but antigenic lipopolysaccharide to hemolysin protein. The conjugates were demonstrated to be covalently coupled by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and sodium deoxycholate detergent gel chromatography.;The vaccinated pigs showed a lower mortality, increased weight gain and lower number of lung lesions as compared to the unvaccinated pigs, strongly suggesting that the conjugate vaccines provide significant protection against challenge exposure to A. pleuropneumoniae.;Following booster immunization given fourteen days after the initial vaccination, antibodies produced to each component of the conjugate vaccines were found to be of high titer. Thus, antibodies to each of these virulence factors are present in the vaccinated pigs prior to challenge as contrasted to unvaccinated pigs. Seven days after challenge, the antibody titers detected in the vaccinated pigs were greatly increased and were about two to three times those of the unvaccinated pigs. The antibodies present in the vaccinated pigs prior to challenge are ready to neutralize the effects of the virulence factors and protect the pigs against this disease. In the unvaccinated pigs, it requires time for the antibodies to be produced after challenge exposure and by this time the infection is severe and can lead to the death of the pigs or the development of lung lesions in surviving pigs.;Antibodies raised against the capsular polysaccharide and lipopolysaccharide components of the conjugate vaccines were found to act as opsonins and enhance the phagocytosis of the bacteria by neutrophils, whereas antibodies to the hemolysin protein component only marginally enhanced phagocytosis. The antibodies to either the capsular polysaccharide, lipopolysaccharide or hemolysin protein did not interact with complement leading to bacteriolysis. Antibodies to the hemolysin protein were found to neutralize the ability of the hemolysin protein exotoxin to lyse neutrophils.